HUNTSVILLE, Texas (AP) – The U.S. Supreme Court will be asked to halt Thursday’s execution of a Texas serial killer whose attorneys are challenging the state’s refusal to release information about where it gets its lethal injection drug.

Lawyers for Tommy Lynn Sells are making the last-minute plea after a federal appeals court allowed the execution to stay on schedule. A lower court had stayed the execution Wednesday, ordering Texas to reveal more information about its drug supplier, but the ruling was quickly tossed on appeal.

Sells, who was sentenced to death for fatally stabbing a 13-year-old South Texas girl in 1999, claims to have committed as many as 70 killings across the U.S. The 49-year-old is scheduled to be lethally injected Thursday evening in Huntsville.

Sells’ attorneys argue that they need to know the name of the company now providing the state with pentobarbital, the drug used during executions, in order to verify the drug’s quality and protect Sells from unconstitutional pain and suffering.

But 5th U.S. Circuit Court of Appeals sided with Texas prison officials, who argued that information about the drug supplier must be kept secret to protect the company from threats of violence. It also found that the stock of the pentobarbital, a powerful sedative, falls within the acceptable ranges of potency.

The court said that had Texas wanted to use a drug never used before for executions or a completely new drug whose efficiency or science was unknown, “the case might be different.”

It’s unclear how the Supreme Court would rule. Last month it rejected similar arguments from a Missouri inmate’s attorneys who challenged the secrecy surrounding where that state obtained its execution drugs, and the condemned prisoner was put to death.

Questions about the source of execution drugs have arisen in several states in recent months as numerous drugmakers – particularly in Europe, where opposition to capital punishment is strongest – have refused to sell their products if they will be used in executions.

That’s led several state prison systems to compounding pharmacies, which are not as heavily regulated by the U.S. Food and Drug Administration as more conventional pharmacies.

A batch of pentobarbital that Texas purchased from a compounding pharmacy in suburban Houston expired at the end of March. The pharmacy refused to sell the state any more drugs, citing threats it received after its name was made public.

That led Texas to its new, undisclosed suppler.

The court case challenging the state’s stance also included 44-year-old Ramiro Hernandez-Llanas, who is scheduled for execution next week. But the 5th Circuit ruling affected only Sells.

Maurie Levin, an attorney for the inmates, said Sells’ case would be appealed to the U.S. Supreme Court.

Levin said the lower court ruling, which had ordered the Texas Department of Criminal Justice to give defense attorneys details about the drug supplier and how the drug was tested, “honors the importance of transparency in the execution process.”

If Sells’ execution is carried out Thursday, it would be the fifth lethal injection this year in Texas, the nation’s busiest death-penalty state.

Sells had dubbed himself “Coast to Coast,” a nod either to his wandering existence as a carnival worker or to his criminal history. Court documents said he claimed as many as 70 murders in his lifetime in states including Alabama, California, Arizona, Kentucky and Arkansas.

In Texas, Sells was convicted of killing 13-year-old Kaylene Harris and nearly killing her 10-year-old friend as they slept in Harris’ home in Del Rio, about 150 miles west of San Antonio, on New Year’s Eve 1999. Sells repeatedly slit Harris’ neck and stabbed her 16 times, and used the same 12-inch boning knife to slash the neck of her friend, Krystal Surles, who survived and helped police find Sells, according to court documents.

Sells was arrested two days later, and his confession was videotaped. A jury convicted him of capital murder in September 2000.

Harris’ father, Terry Harris, said he and his family had befriended Sells at a community church.

In 2003, Sells was indicted but never tried for the slaying of 13-year-old Stephanie Mahaney in Missouri. He also pleaded guilty to capital murder in the 1999 death of 9-year-old Mary Bea Perez, who was strangled during an outdoor festival in Texas. Prosecutors waived the death penalty in exchange for the plea.

Among his other confessions was the slaying of an Illinois family in 1987. Those victims included Ruby Dardeen, who eight months pregnant. Her fatal beating forced her to prematurely give birth. The newborn was killed along with her 3-year-old sibling